1. Field of the Invention
The present invention relates to a wireless communication method using orthogonal frequency division multiplexing (OFDM) and an OFDM transmitter and receiver thereof.
2. Description of the Related Art
In an environment where a plurality of signals arrive with different propagation delay time, such as in a multipath channel, intersymbol interference becomes a major factor of deteriorating communication quality. An orthogonal frequency division multiplexing (OFDM) is known as a scheme to reduce the effect of intersymbol interference which occurs when receiving signals with different propagation delay times.
Generally, in a wireless receiver, a quadrature orthogonal demodulator is used when converting a received high frequency band signal into a baseband signal by frequency conversion. When the quadrature demodulator consists of an analogue circuit, IQ imbalance occurs in the quadrature demodulator due to the imperfection of the analogue circuit. The IQ imbalance is an amplitude error and phase error which occur between an inphase component and an orthogonal component. When the IQ imbalance occurs in the quadrature demodulator, two subcarrier signals among the OFDM signal subcarriers, which are symmetric with respect to a center frequency, interfere with each other. Such mutual interference significantly deteriorates receiving quality.
To compensate IQ imbalance, i.e., to eliminate mutual interference caused by IQ imbalance between the subcarriers positioned symmetric with respect to a center frequency, a technique is proposed in which an amplitude and phase change and mutual interference quantity are estimated for each subcarrier, and the compensation is performed based on such estimation result (JP-A 2001-119364 (KOKAI)). In this technique, in order to estimate the amplitude and phase change caused by IQ imbalance and the mutual interference quantity between subcarriers positioned symmetric with respect to a center frequency, a preamble signal for channel estimation having a particular structure is transmitted.
In the technique described in JP-A 2001-119364 (KOKAI), to estimate the amplitude and phase change which differs for each subcarrier and the mutual interference quantity between the subcarriers positioned symmetric with respect to a center frequency, it is necessary to transmit across all subcarriers a particularly structured channel estimation preamble which doubles the usual length. Therefore, an overhead of the preamble signal for channel estimation is large.